Ion sources which have been used in the past for ion implantation, particularly for ions of the rare earths, cannot produce high-current, metal-ion beams above about ten milliamperes. These known ion sources also produce a large amount of impurity ions and, therefore, the use of mass analysis is necessary for purifying the ion beam. Such purification processes substantially increase the costs and lower the production rates for finished materials.
Heretofore, a common ion source was one typified as being of the magnetron type. Magnetic fields are featured in many ion source designs to help create stable plasmas and to increase the ionization efficiency of the ions within the source. As a rule, the electrons are held in an oscillating or circulating motion between an anode and cathode until they lose an appreciable part of their energy by excitation or ionization collisions. Subsequently, the electrons reach the anode and thus form the anode current of the source. The ions generated in ionizing collisions, after extraction from this source by an extracting field, form the ion beam current.
In one magnetron type of ion source, referred to as the Harwell-Freeman source, the ion source reaches a temperature of about 1100 degrees centigrade. Thus, refractory metals must be ionized in the form of their volatile chlorides. It is usually essential to use completely anhydrous chlorides for ion source operation. Even small traces of water will prolong the source outgassing and in many cases will result in the conversion of some of the chloride to involatile oxychloride or oxide on heating in vacuum. The polyvalent chlorides present a more serious problem because of the difficulties of chemical synthesis and because many of them are extremely hydroscopic. Most of these chloride vapors are very reactive at elevated temperatures which strongly influence the life and reliability of the constructional components which form the apparatus.
The foregoing known method of ionization has several disadvantages for use in ion implantation, namely, the method produces only a few milliamperes of current; the variety of ionic species formed in this source makes magnetic separation of the beam indispensable thus increasing the costs associated with implantation applications; and many physical parameters intervene in the operation of such a source making it difficult to control and limiting its operational life.